payout: say what actually failed instead of blaming transaction size

SendMoney's "oversized transaction that requires a transaction fee of
0.00" is not an oversized transaction and there is no fee. That branch is
taken when nValue + nFeeRequired > GetBalance() -- an empty spendable
balance -- and the fee reads 0.00 because CreateTransaction failed before
setting one. 3,267 of these in debug.log sent me looking for a
transaction-size problem that does not exist. Report what is short, by how
much, and against what balance.

Name COINBASE_SPENDABLE_DEPTH in main.h and derive GetBlocksToMaturity
from it, because the distinction it captures is genuinely surprising:
CMerkleTx::GetCredit values a mined coinbase at zero until
COINBASE_MATURITY+20 deep, not COINBASE_MATURITY.

I first changed the payout gate to that deeper value, on the theory that
each payout was firing 20 blocks before its own coinbase could be spent.
Measuring it refuted that: across 934 payments, 96% succeeded at depth
100, the moment they were queued, which is only possible because a payout
is funded from whatever coin is already mature rather than from its own
block. The gate stays at COINBASE_MATURITY, with a comment recording the
measurement so the next person does not re-make the same change.

The real shortfall is structural and not addressed here: the pool pays out
~99% of each block's subsidy while only matured coinbases are spendable,
so it runs with almost no buffer, and any orphaned block it queued a
payout for drains that buffer permanently. The 24h retry loop absorbs this
-- 0 payouts forfeited -- but the buffer is what needs fixing.

(cherry picked from commit e28df01f90d621b0cd818f28d70a20b8ab5d1a8d)

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